Current government regulations have significantly increased the required fuel efficiency for the automobile industry. This has become prevalent in the development of hybrid vehicle systems and more efficient engine designs. Another area that can be exploited for the reduction of fuel consumption is in the area of vehicle aerodynamics. At speeds above 50 mph, aerodynamic drag becomes the leading negative force acting on the vehicle. It is therefore a viable option to explore drag reduction systems to increase a vehicle's fuel economy. Passenger vehicles produce similar drag characteristics which are characterized by large areas of flow separation at the rear of the vehicle.
Similar to automobiles, bluff bodies share a drag profile with a large area of flow separation at the rear of the body. In order to better understand the air flow characteristics of an automobile, a simplified geometry now referred to as an “Ahmed body” has become a standard benchmark for automotive aerodynamic studies. The developers of this profile, Ahmed et al., discussed the flow characteristics of a bluff-body with rear slant angles between 0° and 90°.
Many studies have been conducted to reduce drag on Ahmed and Ahmed-like bodies using passive control techniques by changing the shape of or adding appendages to the vehicle. For example, a prior study by Verzicco et al. used large-eddy simulation (LES) to examine the effect of devices attached to the base of the vehicle on drag, achieving a 31% drag reduction using a boat-tail device. Another study by Beaudoin and Aider used moving flaps attached to the rear edges of the Ahmed body and found that some configurations achieved a 25% drag reduction.
Further, Gillieron et al. experimentally and analytically investigated the effect of slant angle on the swirling structures in the wake. Using a roughness array placed on the roof of the Ahmed body to produce streaks, Pujals et al. reduced the drag by 10%. Fourrie et al. used an automotive deflector and achieved drag reduction of 9%. In another approach, Bruneau et al. placed porous material on the rear of a square-back Ahmed body and found drag reduction up to 37%. More recently, Thacker et al. changed the shape of the rear slant and found that this reduced drag by 10%.
Such passive techniques, however, typically result in visually unacceptable modifications and appendages to the vehicle bodies, and thus the automotive industry has turned to active drag reduction techniques. Many active flow control techniques such as jets, pulsed jets, and devices to create suction are currently being implemented and characterized. For example, Brunn and Nitsche used diffusers to induce periodic forcing in order to control the flow separation over the slant of the Ahmed body, but only reported on velocity fluctuations and vortex shedding frequencies, and not drag coefficients. Roumeas et al. used a Lattice Boltzman method (LBM) to numerically determine the flow field around Ahmed-like car bodies and investigate suction where a drag reduction of 17% was achieved, and blowing where a drag reduction of 29% was obtained.
Other investigators, such as Krentel et al. used periodically-blowing compressed air actuators on the Ahmed body and achieved a total drag reduction of 5.7%. Aubrun et al. used a steady microjet array along the slant angle of the Ahmed body to reduce the drag coefficient by 14%. Littlewood and Passmore studied the effect on drag of the angle of attack of a slot jet located at the top edge of a square-back vehicle. They found that a jet pointed downwards from the horizontal was the only configuration to reduce the drag. Most recently, Joseph et al. used pulsed jets to achieve an 8% drag reduction. Finally, Bruneau et al. investigated a hybrid passive-active control strategy for reducing drag by using porous layers below the body and active jets. These investigators achieved a maximum drag reduction of 26%.
There remains a need for additional wake control systems for optimization thereof.